TBCs provide protection of superalloy substrates against harsh environments in gas turbine engines. Typical TBCs consist of two main coating layers called bond and top coat. Recently, zirconates have ...become attractive top coat materials as an alternative to yttria stabilized zirconia (YSZ) due to their superior thermal properties. Zirconates are deposited on YSZ layer to decrease the thermal conductivity and to minimize the oxygen penetration. Furthermore, their high melting point as well as phase stability and sintering properties render them promising materials for TBC applications. In this regard, YSZ and YSZ/La2Zr2O7 top coats were deposited using EB-PVD technique on HVOF-CoNiCrAlY bond coated Inconel 718 superalloy, in the present research. TBCs were exposed to isothermal oxidation tests in high temperature furnace at 1000 °C with different time periods and also, they were subjected to furnace cycling tests at 1150 °C. After oxidation and thermal cycling tests, formation of thermally grown oxide (TGO) layers at the interface and crack surfaces were investigated according to analysis results. The results show that double layered TBC system exhibits better oxidation performance in terms of TGO growth and thermal cyclic lifetime compared to single layer TBC system.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Nimonic 80A alloy has been subjected to aluminizing process to improve its high temperature oxidation and wear properties. Pack aluminizing process was carried out for 3 and 5 h at 650 and 700 °C ...temperatures without using a protective atmosphere. Aluminising treatment was carried out using metallic aluminium, ammonium chloride (NH4Cl) and alumina as a filler. The morphology and structure of aluminide layers was analysed by SEM-EDS. It was observed that there was a super bonding between matrix and coating layers which are smooth, dense and porosity free. Dominant phases of Ni2Al3 and Al80Cr20 were detected by XRD analysis. Layer thickness was measured from the surface to the matrix and changed from 20 to 70 μm which was increased with increasing process duration and temperature. The matrix hardness is 400 HVN while the hardness of coating layer raised to 1025 HVN with the process time and temperature. Wear losses decreased with the increase in coating temperature and time in both high temperature and room temperature tests. After isothermal oxidation tests at 1000 °C up to 75 h, all aluminized samples consist of alumina scale while Nimonic 80A consist of Cr2O3 and TiO2 mixed oxides with high thickness. The increased aluminizing temperature and time provide better protection against high temperature oxidation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ
In the present study, an equimolar HfNbTaTiZr refractory high entropy alloy was subjected to powder-pack aluminizing at 950°C for 4 h. High-temperature isothermal oxidation tests were carried out in ...open air on both as-cast and aluminized samples at 1000°C for the durations of 5, 25, and 125 h. Microstructural analysis and oxidation kinetics revealed the formation of a complex oxide layer consisting of (Hf,Zr)O2 and Al2O3 with AlN inclusions in the aluminized samples following oxidation. Significant cracking of the complex oxide layer occurred after the prolonged exposure of 125 h. The as-cast samples followed similar oxidation kinetics under all the conditions. However, the magnitude of mass gain for the aluminized samples was 1.18, 1.55, and 3.60 times less than the as-cast samples after 5, 25, and 125 h of oxidation, respectively. The lower mass gain in the aluminized samples indicated the formation of an adherent oxide layer on the surface of the alloy and that the molar volume of the oxide equaled or exceeded the molar volume of the alloy. As a result, the aluminized alloy was shielded from direct contact with gaseous oxygen and oxidation rate was governed by diffusion through the oxide layer.
•Powder-pack aluminizing refractory HfNbTaTiZr high entropy alloy at 950 °C for 4 h.•A compact coating layer composed of AlN, Al2O3 and NbZrAl6 phases.•Powder-pack aluminizing improved the oxidation resistance of HfNbTaTiZr at 1000 °C by up to 3.6 times.•Oxygen solubility in both RHEA and aluminized RHEA led to the pest oxidation trend.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ
Oxidation is an important phenomenon for materials used at high temperatures. Generally, nickel based superalloys are preferred as metallic materials due to their superior mechanical properties for ...high temperature applications. Inconel 718 has wide usage fields in high temperature applications. In present study, Inconel 718 and Inconel 718/Yttria (%8 Y2O3) Stabilized Zirconia (YSZ)/ Gd2Zr2O7 (GZ) was oxidized in open air atmosphere at 1000 °C for 8, 24 and 100 h. Isothermally oxidized samples were investigated using Scanning Electron Microscopy (SEM), x-ray Diffraction (XRD) and energy dispersive x-ray spectrometry (EDX)-SEM. Results show that Inconel 718 is not suitable for long periods at 1000 °C. Ceramic coated Inconel 718 exhibit better performance and it assist to see phase distributions in microstructure.
Purpose
Thermal barrier coatings (TBCs), which are used in high temperature applications of gas turbines, are damaged due to fuels and airborne minerals under working conditions. Stable zirconia ...coatings, which are usually used as topcoat materials in TBCs, are damaged by interacting at high temperatures with elements such as vanadium and sulfur from low quality fuels. The purpose of this paper is to see the failure mechanism of TBC systems after hot corrosion damages.
Design/methodology/approach
CoNiCrAlY metallic bond coatings of TBC samples were produced by cold gas dynamic spray method which is a new trend production method and stabilized zirconia ceramic top coating was produced by atmospheric plasma spray method. In total, 50% by weight of V
2
O
5
and 50% Na
2
SO
4
salt mixtures were placed on TBC samples and subjected to hot corrosion test at 1000°C.
Findings
Hot corrosion behaviors of TBC samples were examined by scanning electron microscopy, elemental mapping analysis, energy dispersive X-ray spectrometry analysis and X-ray diffraction analysis. TBC samples were damaged at the end of 12-h cycles.
Originality/value
The paper provides to understand the mechanism of hot corrosion of TBCs with cold sprayed metallic bond coat.
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